What Did Jefferson Mean by the Phrase "Wall of Separation"?

When considering Jefferson’s famous letter to the Danbury Baptists, most people only consider how the phrase “wall of separation” sounds to our modern ears.  To us, this phrase sounds as if it is describing an impenetrable impasse which stands between our nation’s religious institutions and her political institutions.  Consider, for example, the following opinion of Supreme Court Justice Felix Frankfurter in McCollum v. Board of Education:

Separation means separation, not something less. Jefferson's metaphor in describing the relation between Church and State speaks of a "wall of separation," not of a fine line easily overstepped.

Justice Frankfurter’s opinion sounds perfectly reasonable to most of those living in the twenty-first century, but it is not consistent with the way that this phrase was understood by our forefathers.
 
The phrase "wall of separation" has a very lengthy history in the Judeo-Christian world view.  It is a reference to the wall which separated between the Jewish and the Gentile worshipers in the temple at Jerusalem, and in Ephesians 2:14, Paul refers to this wall being symbolically broken down by Christ when He died on the cross.  This is almost the exclusive usage of this phrase in the literature prior to Jefferson's letter, and an example of it can be seen in the 1756 edition of The Family Expositer by Philip Doddridge:
 
For he is the Procurer of our Peace, who hath reconciled us, whether Jews or Gentiles, to God and to each other, and hath so incorporated us into one Church, that it may properly be said, he hath made both one, as to an Interest in the Favour of God, and in the Privileges of his People; and that no Difference might remain between us, he hath thrown down the middle Wall of Separation, which divided us from each other, as the Wall which runs between the Court of the Gentiles and that of Israel in the Temple at Jerusalem, divided the Gentile Worshippers from the Jewish.
 
Gentile proselytes to the Jewish religion were not permitted into the inner court of the temple unless they actually became Jews by being circumcised in accordance with Exodus 12:48.  These proselytes were allowed to worship God and to participate in the ceremonies, but they had to remain distinct from the Jews by staying on the Gentile side of the wall of separation.
 
In the Christian era, following Paul's symbolic usage, the term "wall of separation" came to be used as a figure of speech for anything which prevented complete union between two groups.  This usage can be seen with great clarity in James Durham's Dying Man's Testament to the Church of Scotland published in 1740.
 
In such Practices as are opposite and infer Division in the Cases mentioned, there can be no Union or Communion expected, as we see in all the Cases where such have been practised, as of the Novatians, Donatists, and such like; there may be more or less Heat and Bitterness betwixt Men that differ so:  But there cannot be Union, because such Determinations and Practices do draw a Line, and build a Wall of Separation betwixt the one and the other, and so makes one Side to be accounted as not of the same Body.
 
This phrase was also used in this sense in William Hale's "Survey of the Modern State of the Church of Rome" published in The Analytical Review in 1790.
 
The grand pillar of the Romish church was indirectly sapped by its rational members, when they found themselves obliged, by cogent reasons, and the humane suggestions of their own minds, to soften tenets they could not enforce or excuse.  The wall of separation thus removed, all conscientious christians may meet and agree, in observing the main doctrines of the gospel, justice, mercy and truth, leaving rancorous disputes to those who are hearers, rather than doers of the law.
 
But uses of this phrase were not limited to religious writings.  It was also used on multiple occasions to describe King James' successful union of England and Scotland.  One of the more famous of these is found in Sir Francis Bacon's address in the British Parliament:
 
His majesty is the first (as you noted it well) that hath laid lapis angularis, the corner stone of these two mighty kingdoms of England and Scotland, and taken away the wall of separation: whereby his majesty is become the monarch of the most puissant and military nations of the world.
 
And, of course, I cannot fail to mention that Benjamin Franklin once used this phrase to refer to the imaginary boundary between fresh water and salt water at the mouth of a river:
 
In such cases, the salt water comes up the river, and meets the fresh in that part where, if there were a wall or bank of earth across, from side to side, the river would form a lake, fuller indeed at some times than at others, according to the seasons, but whose evaporation would, one time with another, be equal to its supply.
 
When the communication between the two kinds of water is open, this supposed wall of separation may be conceived as a moveable one, which is not only pushed some miles higher up the river by every flood tide from the sea, and carried down again as far by every tide of ebb, but which has even this space of vibration removed nearer to the sea in wet seasons, when the springs and brooks in the upper country are augmented by the falling rains, so as to swell the river, and farther from the sea in dry seasons.
 
Thus we can see from the historical understanding of this phrase that when Jefferson wrote of the “wall of separation between church and state,” he was not referring to a completely impassible barrier as Justice Frankfurter supposed.  He was using a commonly understood phrase to describe the fact that the First Amendment prevented the church and the state from achieving a complete union in America.  They would always remain distinct entities, and the President of our nation would never be, as Jefferson described it, “the legal head of its church.”  This was the true intent of Jefferson’s claim, and we would be fortunate indeed if this intent were once again to be realized among us today.